Chromite in the Paricutin lava flows (1943–1952)

Small euhedral chromite crystals are found in olivine macrophenocrysts (Fo_80–84) from the basaltic andesites (150 ppm Cr) erupted in 1943–1947, and in orthopyroxene macrophenocrysts of the andesites (75 ppm Cr) erupted in 1947–1952. The majority of the chromite octahedra are 5–20 μm in diameter, and some are found in clusters and linear chains of three or more oriented chromite crystals. The composition of the majority of the chromite grains within olivine and orthopyroxene macrophenocrysts is Fe²⁺/(Fe²⁺+Mg)=0.5–0.6, Cr/(Cr+Al)=0.5–0.6 and Fe³⁺/(Fe³⁺+Al+Cr)=0.2–0.3. The chromite crystals in contact with the groundmass are larger, subhedral, and grade in composition from chromite cores to magnetite rims. Comparison of the composition of chromite with those of other volcanic rocks shows that the most primitive Paricutin chromite is richer in total iron and higher in Fe³⁺/(Fe³⁺+Al+Cr) than primary chromite in most lavas. The linear chains of oriented chromite octahedra are found in olivine and orthopyroxene macrophenocrysts, and in the groundmass. These chromite chains are thought to result from diffusion-controlled crystallization because of the very high partition coefficient (1000) of Cr between chromite and melt. We conclude that chromite was a primary phase in the lavas at the time of extrusion and that magnetite only crystallized after extrusion during cooling of the lava flows. The presence of chromite microphenocrysts in andesitic lavas containing as little as 70 ppm Cr can be explained by dissolved H₂O in the melt depressing the liquidus temperature for orthopyroxene such that chromite becomes a liquidus phase. The influence of dissolved H₂O can also explain the lack of plagioclase macrophenocrysts in most of the lavas and the relatively high partition coefficient (20) of Ni between olivine and melt and the high partition coefficient (40) of Cr between orthopyroxene and melt. The liquidus temperature of the basaltic andesite is estimated to have been less than 1140°C, assuming H₂O>1 wt.%, and the log f_O2 to have been above that of the QFM buffer. The chromite and orthopyroxene liquidus temperature of the andesites, assuming H₂O>1 wt.%, is estimated to have been 1100°C or less. The derivation of the later andesites from the earlier basaltic andesites has been explained by a combination of fractional crystallization of olivine, orthopyroxene and plagioclase, and assimilation of xenoliths. The significantly lower Cr, Ni and Mg of the andesites may have been in part due to the separation of olivine macrophenocrysts plus enclosed chromite crystals from the earlier basaltic andesites.     

Geology and geochemistry of lavas at Nekoma volcano: Implications for origin of Quaternary low-K andesite in the north-eastern Honshu arc, Japan

Abstract Nekoma volcano forms part of the arc axis volcanic array of the North-eastern Honshu arc, Japan, which is commonly characterized by medium-K lava suites. However, Nekoma is exceptional because many of its lavas are low-K. This anomaly has been a matter of debate. Nekoma was active from 1.1 to 0.35 Ma. The volcano consists of thick andesite flows and domes associated with block and ash flow deposits produced during lava dome formation. A horseshoe-shaped collapse caldera was formed at the summit and small lava domes extruded into the caldera. Stratigraphy, published K–Ar ages, and tephrochronology define three stages of volcanic activity, about 1.1 Ma (Stage 1), 0.8–0.6 Ma (Stage 2) and 0.45–0.35 Ma (Stage 3; post caldera stage). Low-K andesites occur in all stages. Extremely low-K andesite was also associated in Stage 2 and medium-K andesite was dominant in Stage 3. In general, lavas changed from low-K to medium-K after caldera formation. Geochemical study of the Nekoma lavas shows that both low-K and medium-K lavas are isotopically similar and were derived from a common source. Adatara and Azuma volcanoes, which lie close to Nekoma, also have both low-K and medium-K andesites. However, Sr isotope ratios or temporal-spatial variations in K-level lava classification vary between the three centers. Comparisons of K suites and Sr isotope ratios with frontal arc volcanoes in North-east–Honshu suggest source heterogeneity existed in both medium- and low-K suites. The K contents of lavas and their Sr isotopes are not simply related. This requires re-examination of models for chemical variation of andesites in arcs.     

Geochemistry of the andesite flank lavas of three composite cones within the Atitlán Cauldron,Guatemala

Three composite cones have grown on the southern edge of the previously existing Atitlán Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlán, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Tolimán are mantled by more silicic lava flows. On Tolimán, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K₂O, Rb, Ba and REE than other Guatemalan andesites. Atitlán’s rocks and late lavas from Tolimán have high Al₂O₃ content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al₂O₃ rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH₂O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism.     

Melt viscosity, temperature and transport processes, Troodos ophiolite, Cyprus

The lava section in the Troodos ophiolite, Cyprus, is chemically stratified and divided into a shallow lava sequence with low TiO₂ content and a deeper lava sequence with high TiO₂ content. We calculate the viscosity at magmatic temperature based on major element chemistry of lavas in Cyprus Crustal Study Project (CCSP) Holes CY-1 and 1A. We find that typical shallow low-Ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high-Ti lavas. This implies that, after eruption on-axis, Troodos low-Ti lavas would have been able to flow down the same slope faster and farther than high-Ti lavas. The calculated lava viscosity increases systematically from the lava-sediment interface to the bottom of the composite Hole CY-1/1A. This suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo Troodos rise may have been responsible for the chemical stratification in the Troodos lava pile. Calculated magmatic temperature and molar Mg/(Mg+Fe), or Mg#, decrease systematically down-section, while SiO₂ content increases. Correlation of Mg# in the lavas with Mg# in the underlying, lower crustal plutonic rocks sampled by CCSP Hole CY-4 shows that the shallow lavas came from a high-temperature, lower crustal magma reservoir which is now represented by high-Mg# pyroxenite cumulates, while the deeper lavas were erupted from a lower-temperature, mid-crustal reservoir which is now represented by gabbroic cumulates with lower Mg#.     

Mineralogy and chemistry of modern orogenic lavas — some statistics and implications

Island arc and continental margin (i.e. western Americas) lavas are divided (based on raw data from literature) into basalts (defined by absence of Ca-poor pyroxene, dominated by quartz-normative tholeiites); basaltic andesites and andesites (subdivided on basis of breaks in SiO₂ histogram and taken as <56% and 56–63% SiO₂; Ca-poor pyroxene present; amphibole and biotite absent); and hornblende (±biotite) lavas, which prove to be mainly relatively silicic andesites. Relative proportions of these types are (576 samples): 23% basalts, 29% basaltic andesites; 30% andesites; 18% hornblende andesites. The compilation emphasizes the dominance of calcic plagioclase (labradorite-anorthite) amongst the phenocryst phases. Pyroxenes are largely augite and hypersthene (En_60–75); olivine (Fo_65–85) is common through all compositions. There is an overall close similarity in chemistry and mineralogy between continental margin and island arc lavas, although small consistent differences are apparent (e.g. K₂O, TiO₂, P₂O₅).Modal data indicate that 70% of lavas are phenocryst-rich (20–60 vol.%), and that phenocryst contents show a bimodal distribution. Statistically and petrologically significant correlations are found between mineralogy and rock chemistry, most notably between total rock Al₂O₃ and modal phenocrystic plagioclase (found in all data groups, except hornblende andesites). This, and related data and correlations, indicate that the majority of orogenic magmas are modified by crystal fractionation (including crystal accumulation) processes dominated by plagioclase, and interpreted to occur under relatively low pressures. Dominance of plagioclase suggests phenocryst precipitation occurs typically in water-undersaturated magmas.     

An outline of the geology and geochemistry,and the possible petrogenetic evolution of the volcanic rocks of the Tonga-Kermadec-New Zealand island arc

The Pleistocene-Recent volcanism of this arc extends nearly linearly NNE from northern New Zealand for some 2800 km. Along its western margin lies an active marginal basin (Lau Basin and Havre Trough) which has its southern termination in the Taupo volcanic zone (TVZ, New Zealand). The New Zealand arc segment is developed within a continental crust, whereas the Tonga-Kermadec segments are developed on a ridge system within the oceanic basin. Submarine morphology suggests that the Kermadec volcanoes represent a less advanced stage of evolution relative to those of Tonga.Magmas erupted within the TVZ are dominantly rhyolitic (≈16,000 km³) with subordinate andesites and rare high-alumina tholeiites and dacites. The Kermadec Islands are dominated by tholeiites and basaltic andesites, with subordinate andesites and dacites. The Tongan Islands are dominated by basaltic andesites, with locally developed andesites and dacites. These Tonga-Kermadec lavas are characterised by subcalcic groundmass clinopyroxenes, whereas the younger group of TVZ andesites contain groundmass hypersthene and augite.Geochemically, the TVZ andesites are systematically enriched (relative to those of Tonga-Kermadec) in “incompatible” elements (e.g. K, Rb, Cs, Ba, light REE, U, Th, Zr, Pb), are less Fe-enriched, and contain more radiogenic Sr and Pb (excepting certain ²⁰⁷Pb/²⁰⁴Pb compositions). The evidence points to crustal equilibration of the TVZ andesites prior to eruption.A complete overlap of major and trace element chemistry (including TiO₂) is observed between the Kermadec-TVZ tholeiites and basaltic andesites, and the ocean floor tholeiites of the Lau Basin. Compared to the Tongan lavas, those of the Kermadecs exhibit a greater degree of chemical variability, also reflected in the greater heterogeneity in their Pb isotopic compositions. Moreover, many of the Tonga-Kermadec basaltic andesites exhibit more depleted “incompatible” trace element abundances than the Kermadec and TVZ tholeiites.The “primary” magmas of this arc are interpreted to be of basaltic andesite type, derived from Benioff zone melting (essentially anhydrous), but extensively modified by low-pressure crystal fractionation processes. The Kermadec tholeiites are explained as products of relatively shallow upper mantle partial fusion induced during the earlier stages of diapiric rise of Benioff zone-derived magmas, which are sufficiently hot to intersect the peridotite solidus. This should result in the production and intermixing of a series of magmas extending from olivine tholeiite to basaltic andesite composition. The voluminous rhyolites of TVZ are interpreted as the products of crustal fusion involving Mesozoic sediments.     

The composition and differentiation of the volcanic rocks of Carriacou,grenadines, West Indies

Tertiary volcanic rocks of Carriacou occupy two-thirds of the island. The volcanics include volcaniclastics, lava flows and dome lavas and range in composition from basalts to andesites. Carriacou basalts fall into two petrographic types (a) clinopyroxene-plagioclase-phyric basalts and (b) olivine microphyric basalts; the latter having higher MgO and lower Al₂O₃ than the clinopyroxene basalts. Both types are unusually rich in mafic minerals compared with Lesser Antilles basalts in general, although similar types have been reported from the nearby island of Grenada. The potash to silica ratios are relatively high and confirm the similarity between Carriacou and Grenada basalts and the differences between these basalts and basalts from other islands of the Lesser Antilles. The basaltic andesites and andesites from Carriacou correspond closely in mineralogical and chemical composition with typical andesites found elsewhere in the Lesser Antilles. The geochemistry of the volcanics shows that the olivine microphyric basalts display tholeiitic affinities whereas the clinopyroxeneplagioclase-phyric basalt, basaltic andesites and andesites are calcalkaline. The compositional gradation in both the geochemistry and mineralogy of these volcanics suggests that fractional crystallization played an important role in the derivation of the various magma.     

Differentiation and magma mixing on Kilauea's east rift zone

The lavas of the 1955 east rift eruption of Kilauea Volcano have been the object of considerable petrologic interest for two reasons. First, the early 1955 lavas are among the most differentiated ever erupted at Kilauea, and second, the petrographic character and chemical composition of the lava being erupted changed significantly during the eruption. This shift, from more differentiated (MgO=5.0–5.7%) to more magnesian (MgO=6.2–6.8%) lava, has been variously interpreted, as either due to systematic excavation of a zoned, differentiated magma body, or to invasion of the differentiated magma by more primitive magma, followed by rapid mixing and eruption of the resulting hybrid magmas. Petrologic examination of several nearvent spatter samples of the late 1955 lavas shows abundant evidence for magma mixing, including resorbed and/or reversely zoned crystals of olivine, augite and plagioclase. In addition, the compositional ranges of olivine, plagioclase and groundmass sulfide are very large, implying that the assemblages are hybrid. Core compositions of olivine phenocrysts range from Fo85 to Fo77. The most magnesian olivines in these samples must have originally crystallized from a melt containing 8.0–8.5% MgO, which is distinctly more magnesian than the bulk composition of the late 1955 lavas. The majorelement and trace-element data are either permissive or supportive of a hybrid origin for the late 1955 lavas. In particular, the compositional trends of the 1955 lavas on plots of CaO vs MgO, and the virtual invariance of Al₂O₃ and Sr in these plagioclase-phyric lavas are more easily explained by magma mixing than by fractionation. The pattern of internal disequilibrium/re-equilibration in the late 1955 spatter samples is consistent with reintrusion and mixing having occurred at least twice, during the latter part of the 1955 eruption. Plagioclase zonation preserves possible evidence for additional, earlier reintrusion events. Least-squares modelling the mixing of early 1955 bulk compositions with various summit lavas±olivine pick the 1952 summit lava as most like the primitive component. The results also indicate the primitive component had MgO=7.5–8.0%, corresponding to liquidus temperatures of 1165–1175°C. The absence of Fe-Ti oxide phenocrysts in the late 1955 lavas implies that the cooler component of the hybrid had T>1110°C. Thus the thermal contrast between the two components may have been as much as 55–65°C, sufficient to produce the conspicuous disequilibrium effects visible in the spatter samples.     

Temporal variation in the lavas of Mayon volcano, Philippines

Chemical and petrographic analyses of 51 sequential lava flows from the central vent of Mayon volcano show cyclical variation. In the two most recent cycles, from 1800 to 1876 and from 1881 to the present, one to three basaltic flows are followed by six to ten andesitic flows. Modal and whole-rock chemical parameters show the most regular cyclical variation; calculated groundmass chemical parameters vary less regularly. There is also a long-term trend, over approximately 1700 years of exposed section, toward more basic compositions.The cyclical variation in modes and the chemical composition of the lavas apparently results from periodic influxes of basaltic magma from depth into a shallow magma system. Fractional crystallization of olivine, augite, hypersthene, calcic plagioclase, magnetite and pargasitic hornblende produces successively more andesitic lavas until the next influx of basaltic magma. Differentiation in a deep zone of magma generation is not excluded by the data, but is more likely responsible for the overall change toward more basic compositions than for the cyclical variation.Three points in a cycle — the beginning of basaltic lavas, the beginning of andesitic lavas and a leveling-off of SiO₂, K₂ O and K₂O/Na₂O values — correspond roughly to the beginning of frequent effusive eruptions (with or without an early Plinian eruption), frequent weak to moderately explosive (Strombolian) eruptions, and less frequent explosive (Vulcanian) eruptions, respectively. Recognition of the current stage in a cycle can give a qualitative indication of the nature of forthcoming eruptions. Changes in several specific parameters may precede basaltic lavas and allow early detection of basaltic influxes. These include minima in the glass inclusion/plagioclase phenocryst and phenocryst/groundmass ratios, vesicularity and groundmass TiO₂, a decrease in hypersthene phenocrysts, and constant values for the whole-rock K₂O/Na₂O ratio. The Mayon area is densely populated, making prediction of eruption type important for safety and land-use planning.     

Interpretation of porosity in dacite lava domes as ductile–brittle failure textures

Pores in dacite lavas of three volcanoes in Japan (Unzen, Yakedake and Daisen) have characteristics unlike vesicles. Most of the pores have ragged walls marked by the protrusion of groundmass crystals and phenocrysts. The shapes of these pores range from irregular to planar. Many pores occur in ‘pressure shadows’ adjacent to phenocrysts. Some pores are crack-like and pass around phenocrysts and have blunt terminations. Most of the pores are interpreted as the result of failure of the magma by flow during ductile–brittle transition. This phenomenon, known as cavitation, is well established in hydraulics and the material sciences. Cavitation of the lavas in this study was promoted by steep slope of emplacement, subtle mechanical contrasts between bands, and strain intensification around rigid crystals and lithic fragments. Cavitation in lava can extract volatiles from magma causing premature embrittlement and weakening. Cavitation is inferred to be critical to the initiation of lava-front pyroclastic flows.     

Petrology of tholeiitic lavas from Tanna Island (New Hebrides): Importance of cumulative processes in Island arc magmatism

Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO₂ contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An₉₅ to An₆₀ in the basalts and An₆₀ to An₅₀ in the andesites, but, within each group, they are not correlated with SiO₂ or Na₂O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO₂ variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.     

Cristobalite in the 2011–2012 Cordón Caulle eruption (Chile)

Cristobalite is a low-pressure high-temperature polymorph of SiO₂ found in many volcanic rocks. Its volcanogenic formation has received attention because (1) pure particulate cristobalite can be toxic when inhaled, and its dispersal in volcanic ash is therefore a potential hazard; and (2) its nominal stability field is at temperatures higher than those of magmatic systems, making it an interesting example of metastable crystallization. We present analyses (by XRD, SEM, EPMA, Laser Raman, and synchrotron μ-cT) of representative rhyolitic pyroclasts and of samples from different facies of the compound lava flow from the 2011–2012 eruption of Cordón Caulle (Chile). Cristobalite was not detected in pyroclasts, negating any concern for respiratory hazards, but it makes up 0–23 wt% of lava samples, occurring as prismatic vapour-deposited crystals in vesicles and/or as a groundmass phase in microcrystalline samples. Textures of lava collected near the vent, which best represent those generated in the conduit, indicate that pore isolation promotes vapour deposition of cristobalite. Mass balance shows that the SiO₂ deposited in isolated pore space can have originated from corrosion of the adjacent groundmass. Textures of lava collected down-flow were modified during transport in the insulated interior of the flow, where protracted cooling, additional vesiculation events, and shearing overprint original textures. In the most slowly cooled and intensely sheared samples from the core of the flow, nearly all original pore space is lost, and vapour-deposited cristobalite crystals are crushed and incorporated into the groundmass as the vesicles in which they formed collapse by strain and compaction of the surrounding matrix. Holocrystalline lava from the core of the flow achieves high mass concentrations of cristobalite as slow cooling allows extensive microlite crystallization and devitrification to form groundmass cristobalite. Vapour deposition and devitrification act concurrently but semi-independently. Both are promoted by slow cooling, and it is ultimately devitrification that most strongly contributes to total cristobalite content in a given flow facies. Our findings provide a new field context in which to address questions that have arisen from the study of cristobalite in dome eruptions, with insight afforded by the fundamentally different emplacement geometries of flows and domes.     

Morphology and propagation styles of Miocene submarine basanite lavas at Stanley, northwestern Tasmania, Australia

Miocene submarine basanite pillows, lava lobes, megapillows and sheet lavas in the Stanley Peninsula, northwestern Tasmania, Australia, are well-preserved in three dimensions. The pillows have ropy wrinkles, transverse wrinkles, symmetrical wrinkles, contraction cracks and three types of spreading cracks on their surfaces, and concentric and radial joints in the interior. The lava lobes have ropy wrinkles and contraction cracks on their surfaces. The megapillows are cylindrical with a smoothly curved upper surface and steep sides, and are characterized by distinct radial columnar joints in the interior. They are connected to pillows that propagate radially from its basal margin. The sheet lavas are tabular and have vertical columnar joints in the interior. The largest sheet lava shows a remarkable gradation from a lower 5-m-thick pillow facies to an upper massive facies. The pillows, lava lobes, megapillows and sheet lavas are inferred to have been emplaced completely below sea level but in a shallow marine environment. Their morphological features suggest that the pillows grew by episodic rupture of a near-solid crust and emergence of hot lava, whereas the lava lobes propagated by continuous stretching of the outer skin at the flow front. The megapillows and sheet lavas were master feeder channels by which molten lava was conveyed to the advancing pillows. The sheet lavas propagated by repeated processes of pillow formation and overriding by an upper massive part. Alternating pillow and massive facies commonly found in ocean-floor drill cores and exposed in cross-section in many subaqueous volcanic successions may have formed by propagation of pillows from the basal margins of advancing sheet lavas.     

Internal differentiation of Kutsugata lava flow from Rishiri Volcano,Japan: Processes and timescales of segregation structures' formation

Internal differentiation processes in a solidifying lava flow were investigated for the Kutsugata lava flow from Rishiri Volcano in northern Japan. In a representative 6-m thick lava flow that was investigated in detail in this study, segregation products darker than the host lavas manifested mainly in the form of pipes (vesicle cylinders) and layers (vesicle sheets), occurring around 0.5–2.3 m and 2.0–4.0 m above the base, respectively. Both the cylinders and sheets are significantly richer in incompatible elements such as TiO₂ and K₂O than the host lavas, which suggest that these products essentially represent residual melt produced during solidification of the lava flow. Field observation and the geochemical features of the lavas suggest that the vesicle cylinders grew upward from near the base of the flow by continuous feeding of residual melt from the neighboring host lavas to the heads of the cylinders. On the other hand, the vesicle sheets were produced in situ in the solidifying lava flow as fracture veins caused by horizontal compression. The vesicle cylinders have a remarkably higher MgO content (up to 8 wt.%) than the host lava (< 6 wt.%), whereas the vesicle sheets display MgO depletion (as low as 3.5 wt.%). The relatively high MgO content of the vesicle cylinders cannot be explained solely by the mechanical mixing of olivine phenocrysts with the residual melt. It is suggested that the vesicle cylinders were produced by the extraction of olivine-bearing interstitial melt from an augite-plagioclase network in the host lava, whereas the vesicle sheets were formed by the migration of the residual melt from a crystal network consisting of plagioclase, augite, and olivine in the host lava into platy fractures. We infer that this selective crystal fractionation for forming the vesicle cylinders resulted from processes in which abundant vesicles rejected from the upward-migrating floor solidification front prevented olivine crystals from being incorporated into the crystal network in the host lava. The vesicle cylinders are considered to have formed in ∼ 1 day after the lava flow came to rest, while relatively large vesicle sheets (> 1 cm thick) appeared much later (after ∼ 9 days). The formation of these segregation products was essentially complete within 20 days after the lava emplacement.     

江西省广丰县李家膨润土矿床成因及找矿标志

广丰县李家膨润土矿床赋存于白垩系上统南雄组第二岩性段酸性火山岩的顶底部。膨润土的主要矿物成分为蒙脱石,矿石结构构造与成矿原岩有关。矿石化学成分与酸性熔岩相近,矿体形态及空间分布受酸性熔岩流相带的控制。因此认为,膨润土矿床成矿原岩为酸性玻质熔岩,矿床是由酸性玻璃质熔岩经水解脱玻化而形成,其成因类型为风化淋滤蚀变岩型矿床。膨润土可自然改型,由原来的钠基膨润土改型成钙基膨润土和偏镁的钙基膨润土。对时代较新的酸性火山岩,在玻质熔岩发育处都有可能找到有利用价值的膨润土矿。由于膨润土常由珍珠岩、沸石脱玻化形成,三者常共生出现,是很好的找矿标志。     

Ignimbrite sequence on Gran Canaria

The Miocene sequence of felsic extrusive rocks of about 1000 m total thickness on Gran Canaria is divided into three units:

1. A lower unit of trachytic rhyolites (lavas, composite flows, ignimbrites) characterized by a phenocryst assemblage of anorthoclase (Or_15–20, wt%), clinopyroxene, hypersthene (amphibole substituted for both in ignimbrites), and Fe/Ti-oxides. The commonest groundmass minerals are anorthoclase and alkali-amphibole, with minor quartz and aegirine.
2. A middle unit of comenditic and pantelleritic ignimbrites characterized by anorthoclase (Or_20–32) and amphibole. Phenocryst minerals restricted to individual flows are Fe/Ti-oxides (several comendites), clinopyroxene, biotite, and sphene. The commonest groundmass minerals are anorthoclase and Tiaegirine, with lesser katophorite, arfvedsonite and quartz.
3. An upper unit of trachvtic and phonolitic ignimbites and lava flows (normative ne rarety exceeding 10%) with nepheline phonolite lava flows becoming increasingly abundant upwards. The ignimbrites have mostly anorthoclase (Or_30-04), and biotite, with rarer Fe/Ti-oxides, hornblende, and clinopyroxene. The commonest groundmass minerals are anorthoclase, aegirine, and alkali-amphiboles, and in some flows nepheline.

The change from Na-rich to K-rich anorthoclase upwards in the sequence supports the conclusion, based on over 50 new stratigraphically controlled chemical analyses that the Na₂O/K₂O-ratio decreases within the sequence. possibly as a result of crystal iractionation processes and this effect is independent of probable loss of Na on post-eruptive crystallization. While hydroxyl-bearing phenocryst minerals are absent from all rocks called lava in the field, they are ubiquitous in the ignimbrites, indicating the importance of P_u2o in the generation of suspension-type cruptions. Compositional gradients must have been particularly pronounced in the small magma chambers that existed beneath Gran Canaria, resulting in a wide range of compositionally zoned or mixed deposits.     

Compound and simple lava flows and flood basalts

Compound lava flows, defined as those lavas which are divisible into flowunits, commonly have a shield-like form and are thought to develop when the rate of extrusion of lava is relatively low.Simple lava flows, defined as those lavas which are not divisible into flow-units, are thought to form when the rate of extrusion of lava is relatively high. A logical definition oflava flow must embrace both simple lava sheets and substantial lava shields (compound lava flows) up to 600 m high. Flood basalt piles include both compound and simple flows, but the most extensive and far-reaching flows are simple and they are believed to form when the rate of extrusion of lava is particularly high.     

Flow banding in basaltic pillow lavas from the Early Archean Hooggenoeg Formation, Barberton Greenstone Belt, South Africa

Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the Hooggenoeg Formation in the Barberton Greenstone Belt exhibit pronounced flow banding. The banding is defined by mm to several cm thick alternations of pale green and a dark green, conspicuously variolitic variety of aphyric metabasalt. Concentrations of relatively immobile TiO₂, Al₂O₃ and Cr in both varieties of lava are basaltic. Compositional differences between bands and variations in the lavas in general have been modified by alteration, but indicate mingling of two different basalts, one richer in TiO₂, Al₂O₃, MgO, FeO^t and probably Ni and Cr than the other, as the cause of the banding. The occurrence in certain pillows of blebs of dark metabasalt enclosed in pale green metabasalt, as well as cores of faintly banded or massive dark metabasalt, suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling. The inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows, while diffusion between bands led to partial homogenisation. The most common internal pattern defined by the flow banding in pillows is concentric. In some pillows the banding defines curious mushroom-like structures, commonly cored by dark, variolitic metabasalt, which we interpret as the result of secondary lateral flow due to counter-rotating, transverse (Dean) vortices induced by the axial flow of lava towards the flow front through bends, generally downward, in the tubular pillows. Other pillows exhibit weakly-banded or massive, dark, variolitic cores that are continuous with wedge-shaped apophyses and veins that intrude the flow banded carapace. These cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows.     

Deux exemples de mélange par brassage mécanique entre laves alcalines contrastées dans le strato-volcan du Cantal (Massif Central Français). Implications volcanologiques

During the entire life of Cantal volcano, the main lava types were emitted during rather separated stages of activity. Basalts are scarce in the trachyandesitic complexes, whereas latites are very uncommon in the basaltic units. In both cases, incomplete mechanical mixing would have been active in their genesis as it is suggested by field and microscopic observations. Sanissage composite dome is formed of poorly mixed acid and basic lavas, with still clearly observable reaction relationship between components. Combe de Saure plug shows a dispersion of tiny (smaller than 1 cm) trachyandesitic relics within a prevalent basaltic groundmass, the result of which being a nearly homogeneous melt, so called « leucobasalt », with D.I. similar to that of a hawaiitic rock. From these observations it is suggested that a large part of intermediate Cantal lavas (D.I similar to those from hawaiites-mugearites-benmoreites) may derive from homogeneized mixing, when new basaltic magma entered the chamber filled with differentiated melt.     

Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California

The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular?? calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab?? regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.